Thermoplastic resins in the powder form have found use in a number of applications where it is either impossible or inconvenient to utilize the more conventional cube, pellet or crumb forms. For example, powdered organic polymeric thermoplastic resins in dry form have been used to coat articles by dip coating in either a static or fluidized bed, by powder coating wherein the powder is applied by spraying or dusting, and by flame spraying. In dispersed form, thermoplastic resin powders have been applied as coatings by roller coating, spray coating, slush coating, and dip coating to substrates such as metal, paper, paperboard, and the like. These powders have also been widely employed in conventional powder molding techniques. Other applications of these powders include paper pulp additives; mold release agents for rubber; additives to waxes, paints, and polishes; binders for non-woven fabrics; and so on.
Prior art processes for converting polyolefins and olefin copolymers from coarser forms such as cubes, pellets, crumb and the like, which forms usually are obtained directly from the synthesis process, into the powder form include mechanical grinding, spray drying, precipitation from solution, and dispersion in aqueous media. Additionally, powders are also obtained by shock cooling molten polymers with cold gases and by fluxing with minor amounts of long-chain fatty acids and fatty amides.
In the mechanical grinding methods, the polyolefin in granular form is passed through a high shear pulverizing device, e.g., a Pallmann grinder, to yield particles of irregular shape. In addition to requiring specially designed equipment and cooling to remove the heat generated, such processes yield powders which are not entirely suitable for fluidization or dispersion application wherein spherical particles are required.
Solution methods such as spray drying or precipitation require that the polymer either be synthesized in solution or be dissolved in a solvent or solvents. Inherent in such processes are difficulties in manipulating the solvents, completing removal of the solvent from the product, and classifying the resultant powders. The powders from such processes are of irregular, somewhat rounded shape, and consequently possess only moderately satisfactory fluidization characteristics.
The dispersion process of the prior art involves subdivision under high shear agitation of a polymer in a liquid medium with the aid of various dispersing agents. From the standpoint of cost and simplicity of operation, water is generally the preferred dispersion medium. The dispersing agents usually comprise an alkali metal soap such as sodium stearate, potassium stearate, etc. A disadvantage of these dispersing agents is that they tend to become ineffective at the elevated temperatures at which relatively high molecular weight polyolefins are sufficiently fluid to be dispersible in water, and thereby fail to produce dispersions. Consequently, such prior art processes have generally been limited to relatively low molecular weight polyethylenes.
Polyethylene may also be pulverized by shock cooling in the liquid or molten state with cold gaseous ethylene (Frielink, U.S. Pat. No. 3,719,648). However, it does not appear that spherical particles are produced.
In U.S. Pat. Nos. 3,422,049 and 3,746,681, and also in U.S. Pat. No. 3,432,483, an improved aqueous dispersion process is described. In these processes, the resin feed is subjected to vigorous agitation in the presence of water and a block copolymer of ethylene oxide and propylene oxide (the Pluronic dispersants of Badische-Wyandotte Co.) as the dispersing agent at a temperature above the melting point of the resin and at a pressure sufficient to maintain the water in an aqueous state until a dispersion is produced and thereafter cooling the dispersion below the melting point of the resin. This dispersion process produces exceedingly fine, spherical resin particles, with a number average particle size on the order of 10 microns or less.
It has now been found that coarser thermoplastic resin powders of substantially spherical particles can be prepared following the technique of the prior U.S. Patents when a lithium salt of a higher fatty acid is employed as the dispersing agent. It was unpredictable and therefore quite surprising to find that while corresponding sodium and potassium salts were ineffective in dispersing the high molecular weight polymers of this invention in water, the lithium salts were effective. Moreover, since the lithium salts produce larger particles than the Pluronic dispersants, the use of lithium salts presents a method of obtaining larger spherical particles by the process of my prior invention.
The use of higher lithium salts, of course, is not new in polyolefin technology. Drake et al (U.S. Pat. No. 3,484,402) disclose additive compositions for increasing the melt flow and thermal resistance of polypropylene which consist of 4,6-di(4-hydroxy-3,5-di-tert-butylphenoxy)-2-octylthio-1,3,5-triazine and at least one monocarboxylic fatty acid of the metals of Groups I and II, those of Group II being preferred. Arai et al (U.S. Pat. No. 3,803,065) have found that modification of various inorganic powders by mechano-chemical treatment with a soap of a Group I or II metal or of Al or Pb provides an additive that will impart excellent transparency and good anti-blocking characteristics to polyolefins.
Processes for simultaneously dispersing and saponifying ethylene-vinyl acetate (EVA) copolymers to provide particulate hydrolyzed ethylene-vinyl acetate (HEVA) copolymers of high melt index (100-400) are known. In German Democratic Republic (DDR) Patent Specification No. 88,404, there is described a process for simultaneously dispersing and saponifying EVA copolymers employing sodium hydroxide or potassium hydroxide as the saponification agent and an alkyl sulfonate, an acyl derivative of N-methyltaurine, a higher fatty acid soap, an alkaryl sulfonate or a nonionic surface-active agent derived from ethylene oxide as the dispersion agent.
The process described involves saponifying ethylene-vinyl acetate copolymers at elevated temperature and pressure including, as the final step, discharging the reaction mixture at the operating temperature and pressure directly into a quench vessel at atmospheric or subatmospheric pressure. The quench vessel contains water that is stirred during the discharge operation and the rate of discharge of the reaction mixture is regulated by means of a needle valve. Thus, the sudden release of the reaction mixture causing a portion of the reaction medium to vaporize apparently results in formation of the dispersion due to the atomizing effect of the needle valve. This patent also discloses the optional use of dispersants, but it is apparent from the data provided that such dispersants have only a secondary effect, the primary determinant of dispersion being the discharge of the hot reaction mixture to the quenching bath. From the particle size distribution data provided in the disclosure, it is clear that the presence of dispersing agent seems to favor smaller particles, but is not absolutely essential since comparable dispersions are obtained when dispersing agents are not present in the reaction mixture. There is no indication that a dispersion of the polymer occurs in the reaction mixture prior to discharge when dispersing agents are present but the data provided shows that, on discharge, a dispersion is produced in the presence or absence of dispersing agent. Attempts to obtain dispersions of saponified EVA using N-oleolysarconsinate as dispersing agent by merely cooling the reaction mixture without the described discharge step of DDR No. 88,404 have not produced dispersions. Similarly, when arylsulfonate dispersants are employed in lieu of the sarcosinate, no dispersions are obtained when the reaction mixture is cooled. Thus, it must be concluded that dispersion only occurs on discharge.
The dispersed product obtained by the method of DDR No. 88,404 is of fairly large particle size, the heavy majority of the particles being of diameters greater than 0.125 mm, i.e., usually over 80% of the dispersed particles. In addition, the product is composed of irregular particles, with no spherical particles being observed.